1. Field of the Invention
The present invention relates generally to electron beam apparatus and electron microscopy methods.
2. Description of the Background Art
Optical microscopes, the simplest and most popular instruments used to image objects too small for the naked eye to see, utilize photons with visible wavelengths for imaging. The specimen is illuminated with a broad light beam, and a magnified image of the specimen can be observed using an eye piece or camera. The maximum magnification of a light microscope can be more than 1000× with a diffraction-limited resolution limit of a few hundred nanometers. Improved spatial resolution in an optical microscope can be achieved when shorter wavelengths of light, such as the ultraviolet, are utilized for imaging.
An electron microscope is a type of microscope that uses electrons to illuminate the specimen and create a magnified image of it. The microscope has a greater resolving power than a light microscope, because it uses electrons that have wavelengths few orders of magnitude shorter than visible light, and can achieve magnifications exceeding 1,000,000×. In a typical electron microscope, an electron beam is emitted in a vacuum chamber from an electron gun equipped with a thermionic (tungsten, LaB6), thermally assisted (Schottky, ZrO2) or cold field emission cathode. The electron beam, which typically has an energy ranging from a few hundred eV to few hundred keV and an energy spread ranging from few tenths to few eV, is collimated by one or more condenser lenses and then focused by the final objective lens to form a spot that illuminates the specimen. When the primary electron beam strikes the sample, the electrons deposit energy in a teardrop-shaped volume of the specimen known as the interaction volume, which extends from less than few nm to few μm into the surface, depending on the electron's landing energy and the composition of the specimen. Primary electrons can generate elastically scattered electrons, secondary electrons due to inelastic scattering, characteristic Auger electrons and the emission of electromagnetic radiation. Each of the generated signals can be detected by specialized detectors, amplified and displayed on a CRT display or captured digitally, pixel by pixel on a computer.
Scanning electron microscopes, the most widely used electron microscopes, image the sample surface by scanning it with a tightly focused beam of electrons in a raster scan pattern, pixel by pixel. Transmission electron microscopes (TEM) and low energy electron microscopes (LEEM) are projection (as opposed to scanning) electron microscopes, and thus resemble a conventional light microscope. In a TEM or LEEM, the electron gun forms a broad electron beam that is accelerated to typically a few to hundreds of keV and focused by the objective lens. A parallel flood beam then uniformly illuminates the substrate.
The primary electrons scattered by the specimen produce electrons over a wide range of energies, from secondary electrons in the range of a few eV, to hundreds to thousands of eV for characteristic Auger electrons, and near the landing energy for elastically scattered electrons. Electrons with different energies produce different image contrast and can provide comprehensive information about the specimen, including specimen topography, composition, crystalline structure as well as electrical and magnetic properties. In order to obtain detailed information about the chemical composition, interatomic bonding and local electronic states of non-periodic objects such as nanoparticles, interfaces, defects and macromolecules, an energy resolution of 0.2 eV or less is necessary to discern their characteristic electronic states. Effective means for selecting electrons emitted from the sample with a narrow range of energies for imaging are therefore desirable for detailed characterization of specimens.
One approach to selecting electrons for imaging with a narrow range of energies is to use an energy filter based on a magnetic prism, such as the one disclosed in U.S. Pat. No. 4,851,670, which is entitled “Energy-selected electron imaging filter” and which issued Jul. 25, 1989 to inventor Krivanek. This approach employs a single magnetic sector to disperse the electrons according to their energies and a set of multipole lenses to transform the dispersed energy spectrum into an energy-selected image and another set of multipole lenses to correct the image aberrations. However, the large number of electron-optical components can make the system difficult to align and costly. In addition, narrow and adjustable energy-selecting slits are needed in order to achieve high energy resolution. The manufacture of such fine structures with straight and parallel edges is rather complicated and their reliability of operation under heavy electron bombardment is reduced. Further, the large size of this filter and the net non-zero deflection angle introduced by the filter means that it must be attached at the end of an electron column and thus must replicate some of the optical functionality already available in the main column, e.g. variable optical zoom.
Another approach to selecting electrons for imaging with a narrow range of energies is to use an energy filter based on an omega filter, such as the one disclosed in U.S. Pat. No. 4,740,704, which is entitled “Omega-type electron energy filter” and which issued Apr. 26, 1988 to inventors Rose and Lanio. In this approach, 4 deflection regions with uniform magnetic fields are used to deflect the beam along a path that resembles the greek letter omega. The omega filter disperses the electrons according to their energies and is then returned to the same optical axis. This means that the filter can be inserted into the microscope column and switched on when energy-filtered imaging is desirable. However, narrow and adjustable energy-selecting slits are needed in order to achieve high energy resolution. The manufacture of such fine structures with straight and parallel edges is rather complicated and their reliability of operation under heavy electron bombardment is reduced.
There is significant demand in biological and medical research as well materials science for imaging of specimens at high spatial resolution and with analytical capabilities provided by projection electron microscopes with imaging energy-filtering devices.